An optical coherence tomography (“OCT”) apparatus is used to perform micron-resolution, cross-sectional imaging of biological tissue. The OCT device uses a light source to illuminate a sample and an interferometer and detector to measure the path length to a reflector in the sample by interference between the light reflected from the sample and a reference light beam. The quality of an OCT image, and the accuracy and reliability of subsequent biometric measurements, depend on proper alignment of the scan lines and/or B-scans of OCT measurement data from which the image is derived.
The proper alignment of the OCT data requires detection and correction of both decentration and eye movement during the scan. Decentration refers to the misalignment between the axis presumed for the measurement and the actual axis of the imaging system. In the prior art, decentration was determined by making a limited number of measurements of known features along different lines of sight. Positioning adjustments are made until the eye is sufficiently aligned before the imaging scan begins. Eye movement refers to the rotation or translation of the eye during the scan. Rotation here refers to rotation of the eye alone and does not include any rotations due to rotational motion of the head. Translation refers to translation of the eye due to translations of the head, wherein the eye moves as a fixture within the head. Eye movement is known to generate artifacts in the displayed image. Most commonly, the target (patient) is requested to remain stationary and, if necessary, scanning is repeated until a sufficiently stationary scan is achieved, with no apparent or at least manageable eye motion.
In surgical applications, particularly laser surgery, knowledge of the position of the target tissue is necessary. Eye motion has been tracked using images of the eye and tracking a feature of the eye (such as the pupil) by detecting the feature and following it from image to image as described in U.S. Pat. No. 7,044,602. Alternatively, special tracking beams illuminate areas or spots on the eye as in U.S. Pat. No. 7,001,377, tracking eye motion by analyzing reflection from the illuminated spots.
Other previous eye tracking devices derive eye motion from other monitored signals. For example, U.S. Pat. No. 5,517,021 discloses an eye tracking apparatus that detects bioelectromagnetic signals generated by the muscles that move an individual's eye while U.S. Pat. No. 5,422,689 discloses an eye-tracking device that monitors electro-oculogram signals produced by eye motion.
One biometric measurement is illustrated in a Pachymetry map. Pachymetry is a test that measures thickness of the cornea of the eye. Traditionally, pachymetry has been measured using ultrasound, which provides a reading of corneal thickness from Bowman's membrane to Descemet's membrane. A Pachymetry map (“PM”) is a 2D graph or image showing the thickness of the cornea over an area. Recent advances in Optical Coherence Tomography (“OCT”) provide the opportunity to create a PM without requiring the probe-media contact required for ultrasound imaging. Measurement data is collected or received along multiple OCT scan lines that span a 3-D volume and are nominally arranged in multiple planar groups (B-scans). Once the cornea is detected, its thickness can be measured so that, ideally, for each scan line or for some subset of scan lines there is a measurement of the thickness of the cornea along each scan line. Corneal thickness measurements made over scan lines distributed across the cornea can be used to derive a corneal thickness map whose resolution is dependent upon the resolution of the measurement data in the scan lines and the distribution of the scan lines across the eye. Ideally, the corneal vertex is aligned with the center of the scan pattern to avoid decentration errors. In current AC-OCT machines, the center of scan pattern is usually imperfectly aligned with the corneal vertex because of difficulties of attaining and maintaining subject alignment. Decentricity errors are known to create errors in the PM. Studies on PM repeatability reveal that thickness errors of as much as 50 μm can occur in peripheral zones when the corneal vertex is decentered by as little as 1 mm.
While the Pachymetry map is a map of biometric measurements of the corneal thickness in the anterior chamber of the eye, the retinal nerve fiber layer (RNFL) thickness is a biometric measurement in the posterior chamber of the eye. While the techniques described here are generally applied to the anterior chamber, one skilled in the art can apply these techniques to the posterior of the eye and RNFL thickness measurements or measurements of the fovea, for example.
In recent years, requirements for dense scan coverage of the cornea have increased the duration of the exam, thereby compounding the alignment problem. Due to longer scan durations, the probability of eye movement during the scan increases. A method for determining motion during the scan and either automatically eliminating bad scans or correcting the identified bad scans to make them good scans is greatly desired. Furthermore, a method of automatically detecting and correcting decentration is highly desired. Even if the eye were initially properly aligned, after eye movement it is misaligned. Even if it remains stationary after the eye movement, decentration has occurred. Collection of data on the misaligned eye can be used for improved Pachymetry maps if the misalignment can be detected and characterized to the point that the map can be corrected for the decentration. In particular, the present invention satisfies the need to greatly reduce the error in corneal thickness measurement caused by decentration or motion. More generally, the present invention satisfies the need to greatly reduce image artifacts caused by misregistration of scan lines during measurement data collection.
In light of the above, there is a need in the art for method and apparatus to detect eye position from imaging data. For instance, there is a need to minimize the impact that misalignment and eye motion have on OCT imaging scans using only the OCT scan data.